U.S. patent application number 10/700098 was filed with the patent office on 2004-05-20 for fuel cell.
This patent application is currently assigned to MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.. Invention is credited to Hojo, Nobuhiko, Kondo, Yumi, Mino, Norihisa, Tanaka, Aoi, Yamamoto, Taisuke.
Application Number | 20040096729 10/700098 |
Document ID | / |
Family ID | 32289744 |
Filed Date | 2004-05-20 |
United States Patent
Application |
20040096729 |
Kind Code |
A1 |
Tanaka, Aoi ; et
al. |
May 20, 2004 |
Fuel cell
Abstract
A cell according to the present invention is a fuel cell for
generating an electric power by supplying one electrode with a fuel
and the other electrode with an oxidant. In the fuel cell, a
catalyst layer is formed on at least one surface of at least one of
the one electrode and the other electrode. The catalyst layer is a
layer including catalyst particles alone, a layer including a
mixture of the catalyst particles and other particles, or a layer
of a porous film carrying at least the catalyst particles, and a
molecule including an ion-conducting functional group serving as an
electrolyte is chemically bonded to a surface of at least one
selected from the group consisting of the catalyst particles, the
other particles and the porous film. At least one of the electrodes
has a thin film electrolyte, a catalyst and an electron conducting
substance, thereby suppressing the elution of the electrolyte from
the catalyst layer in an electrode part and the accompanying
voltage drop.
Inventors: |
Tanaka, Aoi; (Osaka-shi,
JP) ; Mino, Norihisa; (Osaka-shi, JP) ; Hojo,
Nobuhiko; (Neyagawa-shi, JP) ; Yamamoto, Taisuke;
(Nara-shi, JP) ; Kondo, Yumi; (Osaka-shi,
JP) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Assignee: |
MATSUSHITA ELECTRIC INDUSTRIAL CO.,
LTD.
1006, Oaza Kadoma
Kadoma-shi
JP
571-8501
|
Family ID: |
32289744 |
Appl. No.: |
10/700098 |
Filed: |
November 3, 2003 |
Current U.S.
Class: |
429/482 ;
429/492; 429/493; 429/513; 429/516; 429/524; 429/532 |
Current CPC
Class: |
H01M 4/9083 20130101;
H01M 4/8652 20130101; Y02E 60/50 20130101; H01M 4/926 20130101;
H01M 4/92 20130101; H01M 4/96 20130101 |
Class at
Publication: |
429/040 ;
429/044; 429/042 |
International
Class: |
H01M 004/86; H01M
004/96; H01M 004/90; H01M 004/92 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 5, 2002 |
JP |
2002-321663 |
Claims
What is claimed is:
1. A fuel cell for generating an electric power by supplying one
electrode with a fuel and the other electrode with an oxidant, the
fuel cell comprising: a catalyst layer formed on at least one
surface of at least one of the one electrode and the other
electrode; wherein the catalyst layer is a layer comprising
catalyst particles alone, a layer comprising a mixture of the
catalyst particles and other particles, or a layer of a porous film
carrying at least the catalyst particles, and a molecule comprising
an ion-conducting functional group serving as an electrolyte is
chemically bonded to a surface of at least one selected from the
group consisting of the catalyst particles, the other particles and
the porous film.
2. The fuel cell according to claim 1, wherein the molecule
comprising the ion-conducting functional group has a mean molecular
weight of 40 to 10,000.
3. The fuel cell according to claim 1, wherein the molecule
comprising the ion-conducting functional group comprises at least
one selected from the group consisting of fluorocarbon and
hydrocarbon.
4. The fuel cell according to claim 1, wherein the ion-conducting
functional group is a proton dissociating functional group.
5. The fuel cell according to claim 4, wherein the proton
dissociating functional group is at least one functional group
selected from the group consisting of a phosphonyl group, a
phosphinyl group, a sulfonyl group, a sulfinic group, a sulfonic
group and a carboxyl group.
6. The fuel cell according to claim 1, wherein the ion-conducting
functional group is a hydrogen bondable functional group.
7. The fuel cell according to claim 6, wherein the hydrogen
bondable functional group is at least one functional group selected
from the group consisting of a mercapto group, an ether linkage
group, a nitro group, a hydroxyl group, a quaternary ammonium base
and an amino group.
8. The fuel cell according to claim 1, wherein the chemical bond is
at least one bond selected from the group consisting of a covalent
bond, an ionic bond, a coordinate bond and a metallic bond.
9. The fuel cell according to claim 1, wherein the chemical bond is
a covalent bond formed by an elimination reaction.
10. The fuel cell according to claim 1, wherein the chemical bond
is a bond via an oxygen atom.
11. The fuel cell according to claim 1, wherein the catalyst
particles comprise at least one selected from the group consisting
of platinum, gold, palladium, nickel, rhodium, cobalt, iridium,
osmium and iron.
12. The fuel cell according to claim 1, wherein the catalyst layer
further comprises an electron conductor.
13. The fuel cell according to claim 12, wherein the electron
conductor is carbon.
14. The fuel cell according to claim 1, wherein the catalyst layer
is the mixture of the catalyst particles and the other particles,
and the other particles are an inorganic substance.
15. The fuel cell according to claim 14, wherein the inorganic
substance comprises at least one selected from the group consisting
of silica, alumina, quartz, glass, ceramics and mica.
16. The fuel cell according to claim 16, wherein the inorganic
substance particles have a mean particle diameter ranging from 0.1
to 100 .mu.m.
17. The fuel cell according to claim 1, wherein the porous film has
a porosity ranging from 5% to 95%.
18. The fuel cell according to claim 1, wherein the porous film has
a mean pore diameter ranging from 0.1 nm to 10 .mu.m.
19. The fuel cell according to claim 1, wherein the catalyst layer
has a thickness ranging from 0.1 to 10000 .mu.m.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a fuel cell that uses
directly hydrogen, methanol, ethanol, dimethyl ether, isopropyl
alcohol, ethylene glycol, glycerin, methane, dimethoxymethane and
the like as a fuel and the air, oxygen or ozone as an oxidant.
[0003] 2. Description of Related Art
[0004] A fuel cell generates electric power by an electrochemical
reaction between a fuel capable of generating a hydrogen ion such
as hydrogen and an oxygen containing oxidant such as the air. Its
structure will be described herein. First, catalyst layers are
formed respectively on both surfaces of a polymer electrolyte for
transporting hydrogen ions selectively. Next, on outer surfaces of
these catalyst layers, gas diffusion layers are formed using, for
example, a water-repellent electrically conductive carbon particle
paper that has both fuel gas permeability and electron
conductivity. The catalyst layer and the gas diffusion layer form
an electrode.
[0005] Then, a gas sealant or a gasket is disposed so as to
surround the electrode and sandwich the polymer electrolyte so that
a fuel to be supplied may not leak out and be mixed with the
oxidant. This sealant or gasket is integrated with the electrode
and the polymer electrolyte, thus forming a membrane electrode
assembly (MEA).
[0006] In general, the catalyst layer of the fuel cell is produced
by preparing a paste of a platinum-based precious metal catalyst as
a catalyst with electrically conductive carbon particles such as
carbon black or graphite (a catalyst carrier) and a polymer
electrolyte, and forming a thin film of this paste.
[0007] Currently, "Nafion" (trade name; manufactured by DuPont.),
which is a perfluorocarbon sulfonic acid polymer, is in general use
as the polymer electrolyte. In order to provide the "Nafion" with
hydrogen ion conductivity, it is necessary to humidify it.
[0008] The incoming fuel from an anode side is separated into
hydrogen ions and electrons on the catalyst of the electrode, while
hydrogen ions and electrons that have passed through the
electrolyte react with the oxidant on the catalyst on a cathode
side. At this time, electric energy can be obtained.
[0009] In the case where hydrogen is used as the fuel, the
reactions below occur in the respective electrodes.
[0010] Anode: 2H.sub.2.fwdarw.4H.sup.++4e.sup.-
[0011] Cathode: O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O
[0012] Alternatively, in the case where methanol is used as the
fuel, the reactions below occur.
[0013] Anode:
CH.sub.3OH+H.sub.2O.fwdarw.CO.sub.2+6H.sup.++6e.sup.-
[0014] Cathode: 3/2O.sub.2+6H.sup.++6e.sup.-.fwdarw.3H.sub.2O
[0015] On the catalyst layer inside the electrode, reactants and
products are diffused, and the electrons and the hydrogen ions
move. Thus, the size of a three-phase zone, which is a reaction
point and serves as a passage of each of the fuel, the electrons
and the hydrogen ions, becomes important.
[0016] The area of the three-phase zone is an effective area of the
catalyst. As this area becomes larger, a utilization factor of the
catalyst increases, leading to a higher cell performance. By
including the polymer electrolyte in the catalyst layer as
described above, the reaction area increases.
[0017] Conventionally, attempts have been made to provide a layer
in which the electrode and the polymer electrolyte are mixed and
dispersed at an interface between the electrode and the
electrolyte. A conventional technology has suggested a method of
applying a dispersed solution of the polymer electrolyte and a
mixture of catalyst onto a polymer electrolyte membrane and
hot-pressing with an electrode, followed by reduction of the
catalyst compound, and a method including the reduction, the
application and then the hot-pressing (for example, see JP
62(1987)-61118 B and JP 62(1987)-61119 B).
[0018] Further, there has been a method of forming a porous
electrode, spraying the polymer electrolyte solution on the
electrode, and then hot-pressing this electrode and the polymer
electrolyte membrane (for example, see JP 2(1990)-48632 B and JP
3(1991)-184266 A). There also is a method of mixing powder prepared
by coating a surface of polymeric resin with a polymer electrolyte
into an electrode (for example, see JP 3(1991)-295172 A). Moreover,
there is a method of mixing a polymer electrolyte, a catalyst,
carbon powder and a fluorocarbon resin and forming a film to be an
electrode (for example, see JP 5(1993)-36418 A).
[0019] However, the above-mentioned conventional catalyst layer
uses the polymer electrolyte that is soluble in water and an
alcohol solution such as ethanol.
[0020] When an alcohol such as methanol is used as the fuel, a
reaction occurs such that alcohol:water=1:1. Accordingly, during
power generation, the electrolyte elutes into the alcohol solution,
so that the three-phase zone decreases, lowering the reaction
efficiency, and causing a problem of voltage drop.
[0021] Furthermore, the electrolyte elutes into water generated in
the cathode during power generation and humidifying water necessary
for hydrogen ion conduction, so that the three-phase zone
decreases, lowering the voltage.
SUMMARY OF THE INVENTION
[0022] In order to solve the conventional problems described above,
the present invention provides a fuel cell using a thin film
electrolyte that does not elute into water or alcohol, thereby
achieving an increased area of a three-phase zone in a catalyst
layer so as to obtain a long lifetime and high voltage.
[0023] A fuel cell of the present invention is a fuel cell for
generating an electric power by supplying one electrode with a fuel
and the other electrode with an oxidant. The fuel cell includes a
catalyst layer formed on at least one surface of at least one of
the one electrode and the other electrode. The catalyst layer is a
layer including catalyst particles alone, a layer including a
mixture of the catalyst particles and other particles, or a layer
of a porous film carrying at least the catalyst particles, and a
molecule including an ion-conducting functional group serving as an
electrolyte is chemically bonded to a surface of at least one
selected from the group consisting of the catalyst particles, the
other particles and the porous film.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic view showing a catalyst layer
according to Example 1 of the present invention.
[0025] FIG. 2 is a schematic plan view showing an electrolyte
membrane electrode assembly (MEA) of a fuel cell according to
Example 1 of the present invention.
[0026] FIG. 3 is a perspective view showing a structure of a unit
cell of the fuel cell according to Example 1 of the present
invention.
[0027] FIG. 4 is a sectional view showing a stacked structure in
which the unit cells of the fuel cell according to Example 1 of the
present invention are stacked.
[0028] FIG. 5 is a schematic view showing a catalyst layer
according to Example 2 of the present invention.
[0029] FIG. 6 is a schematic view showing a catalyst layer
according to Example 3 of the present invention.
[0030] FIG. 7 is a schematic view showing a catalyst layer
according to Example 4 of the present invention.
[0031] FIG. 8 is a schematic view showing an electrolyte membrane
electrode assembly (MEA) according to Example 5 of the present
invention.
[0032] FIG. 9 is a schematic view showing an electrolyte membrane
electrode assembly (MEA) according to Example 6 of the present
invention.
[0033] FIG. 10 is a schematic plan view showing an electrolyte
membrane electrode assembly (MEA) according to Example 7 of the
present invention.
[0034] FIG. 11 is a schematic plan view showing an electrolyte
membrane electrode assembly (MEA) according to Example 8 of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] The present invention relates to a fuel cell. In the fuel
cell of the present invention, its catalyst layer is a layer
including catalyst particles alone, a layer including a mixture of
the catalyst particles and other particles, or a layer of a porous
film carrying at least the catalyst particles, and a molecule
including an ion-conducting functional group serving as an
electrolyte is chemically bonded to a surface of at least one
selected from the group consisting of the catalyst particles, the
other particles and the porous film. The ion-conducting functional
group functions as an electrolyte. It is preferable that the
chemical bond is any of a covalent bond, an ionic bond, a
coordinate bond and a metallic bond. In particular, a covalent bond
formed by an elimination reaction is preferable because it is the
most stable in chemical and physical terms. Here, the elimination
reaction refers to dehydrohalogenation, dealcoholization (wherein
the alcohol has 1 to 3 carbon atoms) or the like.
[0036] It is preferable that the molecule including the
ion-conducting functional group has a mean molecular weight of 40
to 10,000. The molecule including the ion-conducting functional
group needs to be directly chemically-bonded to a substrate, and a
molecular weight of at least 40 is necessary for the molecule to
have a functional group needed for that chemical bond. On the other
hand, the molecular weight exceeding 10,000 is not appropriate
because such a large molecular weight makes it difficult to achieve
a conformation that is most suitable for forming a molecular thin
film, and thus, the molecule including the ion-conducting
functional group cannot be bonded to the substrate suitably.
[0037] The molecular weight can be determined by "TOF-SIMS
(Time-of-Flight Secondary Ion Mass Spectrometry)." TOF-SIMS is a
generally-used surface analysis method, which is also detailed in
the documents below.
[0038] (1) Wang, D., et al.: Catal. Today, 12(1992), 375
[0039] (2) Wang, D., et al.: J. Mater. Sci., 28(1993), 1396
[0040] (3) Wang, D., et al.: Compos. Sci. Technol., 50-2(1994),
215
[0041] (4) Toyota Central R&D Labs., Inc., R&D Review Vol.
34, No. 2 (1996.2), 11 (to which reference is to be made in
particular)
[0042] In the above description, the molecular thin film refers to
a film formed by allowing an elimination reaction such as
dehydrochlorination, dealcoholization or deisocyanation between a
reactive group such as a chloro group, an alkoxyl group or an
isocyanate group present at the end of a molecule and an active
hydrogen (a hydroxyl group, a carboxyl group, an amino group, an
imino group or the like) of a substrate, and a film obtained by a
polymerization of the above film. For example, in the case where
the functional group at the end of the molecule is --SiCl.sub.3,
--Si(OR).sub.3 (wherein R is an alkyl group having 1 to 3 carbon
atoms) or --Si(NCO).sub.3, when an active hydrogen included in --OH
group, --CHO group, --COOH group, --NH.sub.2 group, >NH group or
the like is present on the surface of the substrate or that of an
underlying layer formed on the substrate, dehydrochlorination,
dealcoholization or deisocyanation occurs so as to covalently-bond
a chemisorbed molecule to the surface of the substrate or that of
the underlying layer formed on the substrate. The molecular film
formed by this method is called a "chemisorption film" or a "self
assembling film." In order to further polymerize this chemisorption
film, unsaturated bonds are allowed to be present in the molecules
in advance, and the molecules are polymerized by
photopolymerization or the like after forming the chemisorption
film.
[0043] It is preferable that the molecule including the
ion-conducting functional group includes at least one organic group
selected from the group consisting of fluorocarbon and hydrocarbon.
This creates a state in which water molecules cannot pass through
easily and protons can pass through easily. In particular, a
fluorocarbon group ((CF.sub.2).sub.n--, wherein n ranges from 2 to
30) is preferable because it is sufficiently stable at high
electric potentials that the molecule is not cleaved easily, the
protons can pass through easily and the water molecules cannot pass
through easily.
[0044] It is preferable that the ion-conducting functional group is
a proton dissociating functional group.
[0045] It is preferable that the proton dissociating functional
group is at least one functional group selected from the group
consisting of a phosphonyl group, a phosphinyl group, a sulfonyl
group, a sulfinic group, a sulfonic group and a carboxyl group.
[0046] It is preferable that the ion-conducting functional group is
a hydrogen bondable functional group. This allows proton conduction
utilizing water molecules bonded to the hydrogen bondable
functional group, so that an electrode catalyst reaction can be
achieved.
[0047] It is preferable that the hydrogen bondable functional group
is at least one functional group selected from the group consisting
of a mercapto group, an ether linkage group, a nitro group, a
hydroxyl group, a quaternary ammonium base and an amino group.
[0048] It is preferable that the chemical bond is at least one bond
selected from the group consisting of a covalent bond, an ionic
bond, a coordinate bond and a metallic bond. It is preferable that
the chemical bond is a covalent bond formed by an elimination
reaction. This is because the most stable bond can be achieved.
[0049] It is preferable that the chemical bond is a bond via an
oxygen atom. Besides the oxygen atom, the chemical bond may be a
bond via a nitrogen atom. This is because the above-mentioned
elimination reaction occurs between, for example, a chloro group or
an alkoxyl group at the end of a molecule of an organic compound
and an active hydrogen, for example, a hydroxyl group (--OH), a
carboxyl group (--COOH), an amino group (--NH.sub.2), or an imino
group (>NH), on the substrate surface.
[0050] It is preferable that the catalyst particles include at
least one selected from the group consisting of platinum, gold,
palladium, nickel, rhodium, cobalt, iridium, osmium and iron. This
is because they are an excellent oxidation catalyst.
[0051] It is preferable that the catalyst layer further includes an
electron conductor.
[0052] It is preferable that the electron conductor is carbon,
since carbon has an excellent electron conductivity and is
electrochemically stable and its surface has a functional group to
which the molecule including the ion-conducting functional group
can be chemically bonded.
[0053] It is preferable that the other particles are an inorganic
substance. It is preferable that the inorganic substance includes
at least one selected from the group consisting of silica, alumina,
quartz, glass, ceramics and mica. This is because the surface of
the inorganic substance has a functional group to which the
molecule including the ion-conducting functional group can be
chemically bonded, and in particular, the densities of the
functional groups present on the surface of silica and alumina are
higher than those of other inorganic substances. In the above
description, the ceramics may include glass, since there also are
glassy ceramics such as porcelain and pottery.
[0054] It is preferable that the inorganic substance is in the form
of particles.
[0055] It is preferable that the inorganic substance particles have
a mean particle diameter ranging from 0.1 to 100 .mu.m. When the
mean particle diameter exceeds 100 .mu.m, the surface area of the
inorganic substance particles is so small that the molecules having
the ion-conducting functional group are sparse, lowering ion
conductivity. On the other hand, when the mean particle diameter is
smaller than 0.1 .mu.m, the inorganic substance particles are
covered with the catalyst and the electron-conducting substances,
so that ions cannot be conducted to the electrolyte part.
Consequently, the cell voltage drops.
[0056] It is preferable that the porous film has a porosity ranging
from 5% to 95%. The porosity smaller than 5% lowers diffusibility
of the fuel and products, so that power generation becomes
difficult especially in a large current density region requiring a
large amount of fuel (diffusion controlled). On the other hand,
when the porosity is larger than 95%, the electron movement between
the electron-conducting substances and the ion conduction between
the ion-conducting substances become difficult, so that the power
generation becomes difficult especially in a large current density
region.
[0057] It is preferable that the porous film has a mean pore
diameter ranging from 0.1 nm to 10 .mu.m. When the mean particle
diameter is smaller than 0.1 nm, the molecule having the
ion-conducting functional group enters into the pores and it is
difficult to form a chemical bond. On the other hand, when the mean
particle diameter is larger than 10 .mu.m, the ion-conducting
functional groups are spaced away farther than an ion-conductible
distance. As a result, the ion-conducting speed decreases, so that
the speed of the catalyst reaction also lowers.
[0058] It is preferable that the catalyst layer has a thickness
ranging from 0.1 to 10000 .mu.m. The thickness smaller than 0.1
.mu.m makes it difficult to withstand the pressure during cell
production and that for fuel supply. On the other hand, the
thickness larger than 10000 .mu.m lowers the fuel diffusibility, so
that the cell voltage drops.
[0059] The electrolyte of the present invention (hereinafter,
referred to as a "thin film electrolyte") is obtained by, for
example, chemically bonding the molecule including the
ion-conducting functional group to any of the catalyst particles,
the other particles and particles serving as a material of the
porous film, and then compression-molding the particles so as to
form them into a sheet, a plate or a film. Other methods may
include compression-molding the particles in advance so as to form
them into a sheet, a plate or a film, and then chemically bonding
the molecule including the ion-conducting functional group
thereto.
[0060] In accordance with the present invention, since at least one
of the electrodes has the thin film electrolyte, the catalyst and
the electron-conducting substance, it is possible to suppress the
elution of the electrolyte from the catalyst layer of the electrode
part and the voltage drop accompanying therewith.
EXAMPLES
[0061] The following is a specific description of the present
invention by way of examples.
Example 1
[0062] Example 1 is directed to an exemplary case in which a
catalyst layer includes catalyst particles alone.
[0063] Platinum black (HiSPEC1000, manufactured by Johnson Matthey
plc., mean particle diameter: 1.5 .mu.m, catalyst particles serve
as a substrate to which a thin film electrolyte is bonded) burned
at 600.degree. C. in a nitrogen atmosphere and platinum ruthenium
black (HiSPEC6000, manufactured by Johnson Matthey plc., mean
particle diameter: 2.0 .mu.m) treated in a similar manner were used
as a cathode catalyst and an anode catalyst, respectively.
[0064] Molecules containing an ion-conducting functional group
serving as an electrolyte (hereinafter, referred to as a thin film
electrolyte) were chemically bonded to the surfaces of the cathode
catalyst and the anode catalyst, thus forming catalyst layers. The
method for producing the catalyst layers is as follows.
[0065] 1 wt % of trichlorosilane compound:
CH.sub.2.dbd.CH--(CF.sub.2).sub- .14(CH.sub.2).sub.2SiCl.sub.3
containing a vinyl group at its end and a fluorocarbon chain at its
middle part, which was a reactant, was dissolved in a nonaqueous
solvent in which n-hexadecane and chloroform were mixed at 4:1. The
platinum black and the platinum ruthenium black serving as the
catalysts were immersed in this solution for 2 hours.
Dehydrochlorination occurred between a hydroxyl group (--OH)
present on the catalyst surface and a chloro group in the
trichlorosilane compound, so that a monomolecule of the
trichlorosilane compound was bonded to the catalyst surface via
oxygen as shown in the formula (1) below. 1
[0066] After the resultant particles were washed in chloroform,
which was a nonaqueous solvent, so as to remove unreacted
substances, they were allowed to react with water in the air.
Consequently, the monomolecules were bonded to each other via
oxygen so as to form a molecular thin film derived from the
trichlorosilane compound as shown in the formula (2) below. 2
[0067] Next, the catalyst whose surface was provided with the thin
film was allowed to react with a fuming sulfuric acid, whereby an
unsaturated bond (a vinyl bond) at the end of the molecule was
sulfonated, so that a molecular thin film shown in the formula (3)
below was formed. This molecular thin film had a molecular weight
of about 912 and a molecular length of 2.8 nm. Here, a SO.sub.3--
group was a group having an ion conductivity, which was formed
uniformly on the surface of the molecular thin film in the present
embodiment. 3
[0068] The catalyst provided with the thin film electrolyte was
mixed with ion exchanged water and dispersion of
polytetrafluoroethylene (PTFE) (ND-1, manufactured by DAIKIN
INDUSTRIES, Ltd.), thus preparing a paste. When preparing the
paste, the mixed ratio based on weight was ion exchanged
water:catalyst with thin film electrolyte=1:10 and the PTFE
dispersion was 1 wt %. FIG. 1 is a schematic view showing the
catalyst layer in the present example. In FIG. 1, numeral 11
denotes the platinum black or the platinum ruthenium black serving
as the catalyst, and numeral 12 denotes the thin film
electrolyte.
[0069] The electrolyte part used in the present example was
prepared using a thin film electrolyte, whose production method was
as follows. A trialkoxysilane compound
H.sup.+SO.sub.3--(CH.sub.2).sub.2(CF.sub.2).sub.-
14(CH.sub.2).sub.2Si(OCH.sub.3).sub.3 was pressed into pores of a
60 .mu.m thick alumina membrane filter, which was an inorganic
porous body, (outer dimension: 8 cm.times.8 cm) having 0.02 .mu.m
pores. Dealcoholization occurred between a hydroxyl group (--OH) on
the alumina surface and an alkoxy group (in this case, a methoxy
group: --OCH.sub.3), so that a trialkoxysilane compound was bonded
to the alumina as shown in the formula (4). 4
[0070] The monomolecules were bonded to each other by the
dealcoholization, so that the thin film electrolyte was formed in
the pores as shown in the formula (5) below. 5
[0071] Onto both surfaces of the obtained electrolyte part, the
catalyst pastes prepared respectively for the anode and the cathode
were applied in a size of an outer dimension of 5 cm.times.5 cm at
the center of the electrolyte part and dried in an electric furnace
at 50.degree. C., thus forming the catalyst layers. From both of
the outer sides, the catalyst layers were sandwiched as one piece
by water-repellent carbon papers (TGP-H-060, manufactured by Toray
Industries. Inc., outer dimension: 5 cm.times.5 cm), thus forming
electrodes. The portion including the anode electrode, the
electrolyte part and the cathode electrode was called a membrane
electrode assembly (MEA), and the one produced in the present
method was referred to as a MEA 1. FIG. 2 is a schematic view
showing the MEA. Numeral 21 denotes the electrodes, and numeral 22
denotes the electrolyte part.
[0072] An outer peripheral portion of the MEA 1 was sandwiched by
150 .mu.m thick gaskets made of silicone rubber (outer dimension: 8
cm.times.8 cm), then hot-pressed at a gauge pressure of 2.5
Mpa.multidot.g. Further, manifolds for cooling water, fuel and
oxidant flows were formed.
[0073] Subsequently, a separator formed of a 13 mm thick
resin-impregnated graphite plate having an outer dimension of 8
cm.times.8 cm and fuel, oxidant and cooling water channels of 5 mm
were prepared. Two separators were used so that the separator
provided with the oxidant channel was superposed on one surface of
the MEA joined to the gasket plate and the separator provided with
the fuel channel was superposed on the other surface thereof, thus
forming a unit fuel cell 1. FIG. 3 is a schematic view showing the
unit cell. Numeral 23 denotes the MEA, numeral 24 denotes the
gasket plate, numerals 25, 26 and 27 denote the manifolds, and
numeral 28 denotes the separator.
[0074] The fuel and cooling water to the separator 28 were sent to
each cell through the manifolds 25, 26 and 27, flowed through the
channels on the separator 28 and were supplied to the MEA 23.
[0075] FIG. 4 is a sectional view showing how the unit cells
obtained in FIG. 3 were stacked and connected in series. After unit
cells 31 and 32 were stacked, they were sandwiched by separators 33
and 34 provided with cooling water channels 43 and 44, and then
unit cells 35 and 36 were stacked on outer sides of the separators
33 and 34. In this manner, an 8-cell-layered cell stack was formed.
In other words, adjacent cells were connected in series via the
separators 33 and 34. Numerals 37, 38, 39 and 40 each denote a
MEA.
[0076] In this case, both ends of the cell stack were fixed using
stainless steel current collector plates with gold-plated surfaces,
insulator plates formed of an electrically insulating material and
further end plates and fastening rods. The fastening pressure was
1.47.times.10.sup.6 Pa (15 kgf/cm.sup.2).
Example 2
[0077] Example 2 is directed to an exemplary case in which a
catalyst layer includes catalyst particles and electron conducting
particles. The catalyst particles serve as a substrate to which a
thin film electrolyte is bonded.
[0078] After platinum carrying carbon manufactured by Tanaka
Kikinzoku Kogyo K.K. (TEC10E50E, mean particle diameter: 30 .mu.m)
or platinum ruthenium carrying carbon manufactured by Tanaka
Kikinzoku Kogyo K.K. (TEC61E54, mean particle diameter: 30 .mu.m)
was burned at 600.degree. C. in a nitrogen atmosphere, the
trichlorosilane compound described in Example 1 was formed into a
molecular thin film on a catalyst surface according to the method
described in Example 1. By sulfonation thereafter, a thin film
electrolyte was produced on the platinum or platinum ruthenium
catalyst carried on the carbon. This made it possible to produce
the thin film electrolyte formed of an organic silane compound on
the platinum or the platinum ruthenium alloy on the carbon. FIG. 5
is a schematic view showing the catalyst layer. Numeral 51 denotes
the platinum or the platinum ruthenium alloy serving as the
catalyst, numeral 52 denotes the carbon carrying the catalyst, and
numeral 53 denotes the thin film electrolyte.
[0079] The resultant carbon carried catalyst with the thin film
electrolyte was mixed with ion exchanged water and a PTFE
dispersion by a method similar to that in Example 1, thus obtaining
a catalyst paste. At this time, the mixed ratio based on weight was
ion exchanged water:carbon carried catalyst with thin film
electrolyte=5:1 and 1 wt % of the PTFE dispersion was added. The
paste was applied to an electrolyte part produced according to the
method described in Example 1 and formed into one piece with carbon
papers according to the method described in Example 1. In this
manner, a MEA 2 was produced. A unit cell 2 was produced using the
MEA 2 by the method described in Example 1.
Example 3
[0080] Example 3 is directed to an exemplary case in which a
catalyst layer includes a mixed layer of catalyst particles and
other particles. The other particles (in the present example,
silica particles) added to the catalyst layer serve as a substrate
to which a thin film electrolyte is bonded.
[0081] The trichlorosilane compound described in Example 1 was
bonded to the surface of silica particles with a diameter of 100 nm
(error of .+-.15 nm) according to the method described in Example
1, followed by sulfonation, thus producing a thin film electrolyte
on the silica surface. In the above, platinum black (HiSPEC1000,
manufactured by Johnson Matthey plc.) or platinum ruthenium black
(HiSPEC6000, manufactured by Johnson Matthey plc.) was mixed as a
catalyst so that the weight ratio of silica
particles:catalyst=1:10. FIG. 6 is a schematic view showing the
catalyst layer. Numeral 61 denotes the silica particles, numeral 62
denotes the thin film electrolyte, and numeral 63 denotes the
platinum black or the platinum ruthenium black.
[0082] Further, 1 wt % of a mixed solution of ethanol and ion
exchanged water (ethanol: ion exchanged water=4:1) and a PTFE
dispersion (ND-1, manufactured by DAIKIN INDUSTRIES, Ltd.) was
added thereto and stirred ultrasonically at a room temperature,
thus preparing a catalyst layer paste.
[0083] A predetermined sized (5 cm.times.5 cm) masking was provided
on an electrolyte part including the thin film electrolyte produced
according to the method described in Example 1, and the catalyst
paste was sprayed on both surfaces of the anode and the cathode and
formed in one piece with carbon papers by the method described in
Example 1, thus forming a MEA 3. Then, a unit cell 3 was produced
using the MEA 3 according to the method described in Example 1.
Example 4
[0084] Example 4 is directed to an exemplary case in which a
catalyst layer includes a catalyst and an electron conducting
substance and the electron conducting substance (in this case,
carbon black) of the catalyst layer serves as a substrate to which
a thin film electrolyte is bonded.
[0085] Platinum carrying carbon manufactured by Tanaka Kikinzoku
Kogyo K.K. (TEC10E50E) or platinum ruthenium carrying carbon
manufactured by Tanaka Kikinzoku Kogyo K.K. (TEC61E54) was heated
with a fuming sulfuric acid in a nitrogen atmosphere at a
temperature from 55.degree. C. to 60.degree. C. and stirred for 50
hours. This was dropped in absolute ether maintained at 0.degree.
C. so as to obtain a solid. This solid was stirred with distilled
water in a nitrogen atmosphere for 10 hours, and solids obtained
after filtering were dried in a vacuum. Among the resultant solids,
the platinum carrying carbon was used for the cathode, and the
platinum ruthenium carrying carbon was used for the anode.
[0086] According to the method described in Example 1, a molecular
thin film was formed on the surface of each catalyst carrying
carbon that had been treated, followed by sulfonation, thus
obtaining a thin film electrolyte. On the sulfuric acid-treated
carbon surface, a hydroxyl group (--OH group) and a carboxyl group
(--COOH group) were present. Dealcoholization occurred between this
part and a methoxy group of the silane compound, so that a
molecular thin film from the silane compound was formed on the
carbon. FIG. 7 is a schematic view showing this catalyst. Numeral
71 denotes the platinum or the platinum ruthenium alloy serving as
the catalyst, and numeral 72 denotes the carbon carrying the
catalyst. Also, numeral 73 denotes the thin film electrolyte on the
carrier carbon.
[0087] The resultant molecular thin film was mixed with a Flemion
solution FSS-1 (manufactured by ASAHI GLASS CO., LTD., 9 wt %
ethanol solution), which was a perfluorocarbon sulfonic acid
solution, and deionized water so as to prepare a paste, which was
then applied onto both surfaces of the electrolyte produced by the
method described in Example 1, dried and formed in one piece with
carbon papers according to the method described in Example 1, thus
obtaining a MEA 4. Then, a unit cell 4 was produced using the MEA 4
according to the method described in Example 1.
Example 5
[0088] Example 5 is directed to an exemplary case in which a
catalyst layer includes a catalyst and an electron conducting
substance, the electron conducting substance (in this case, carbon
black) of the catalyst layer serves as a substrate to which a thin
film electrolyte is bonded and the electrolyte and the catalyst
layer are formed in one step.
[0089] 50 wt % platinum having a mean particle diameter of 3 nm
carried by Ketjen Black EC (trade name; manufactured by AKZO
Chemie, the Netherlands) having a mean primary particle diameter of
30 nm was used as a cathode catalyst, while 25 wt % platinum having
a mean particle diameter of 3 nm and 25 wt % ruthenium having a
mean particle diameter of 3 nm that were carried by the same Ketjen
Black EC were used as an anode catalyst.
[0090] This carbon carrying the catalysts, a PTFE dispersion and
ion exchanged water were mixed, filtered, and then formed into a
sheet by roller pressing. Silica sol having a mean particle
diameter of 80 nm was applied thereto and dried, followed by
burning in an argon gas at 500.degree. C.
[0091] After burning, a trialkoxysilane compound
H.sup.+SO.sub.3--(CH.sub.-
2).sub.2(CF.sub.2).sub.14(CH.sub.2).sub.2Si(OCH.sub.3).sub.3 was
pressed into the surface to which the silica sol had been applied.
Dealcoholization occurred between a hydroxyl group (--OH) on the
silica sol surface and an alkoxy group (in this case, a methoxy
group: --OCH.sub.3), so that the trialkoxysilane compound was
bonded onto the silica. These monomolecules were bonded to each
other, thus forming an electrolyte part.
[0092] The silane compound was filled in not only the silica sol
portion but also a solidified catalyst portion, making it possible
to produce the catalyst layer and the electrolyte part at one time.
This was formed in one piece with carbon papers by the method
described in Example 1 so as to form a MEA 5. FIG. 8 is a schematic
view thereof. Numeral 81 denotes the catalyst layer portion, and
numeral 82 denotes the electrolyte part. Numeral 83 denotes the
catalyst, numeral 84 denotes the carbon carrying the catalyst, and
numeral 85 denotes the thin film electrolyte formed on the carbon.
Also, numeral 86 constituting the electrolyte part indicates the
silica sol, and numeral 87 indicates the thin film electrolyte.
[0093] Then, a unit cell 5 was produced using this MEA 5 according
to the method described in Example 1.
Example 6
[0094] Example 6 is directed to an exemplary case in which a
catalyst layer includes catalyst particles, an electron conducting
substance and other particles, and the added other particles (in
this case, alumina particles) serve as a substrate to which a thin
film electrolyte is bonded.
[0095] Alumina particles having a particle diameter of 100 .mu.m
were burned in a nitrogen atmosphere in an electric furnace at
150.degree. C. for 3 hours and dried, and then immersed in a silane
compound diluted solution described in Example 1 and stirred for 30
hours while being heated at 60.degree. C. After the stirring, the
washing and filtering were repeated using a toluene anhydride
solution, followed by re-drying in a nitrogen atmosphere at a room
temperature.
[0096] The resultant alumina particles and platinum carrying carbon
manufactured by Tanaka Kikinzoku Kogyo K.K. (TEC10E50E) or platinum
ruthenium carrying carbon manufactured by Tanaka Kikinzoku Kogyo
K.K. (TEC61E54) were mixed, to which ion exchanged water and a PTFE
dispersion further were added, thus forming a catalyst paste. This
catalyst paste was formed into a thin film and then formed as one
piece with the electrolyte by the method described in Example 1,
thus producing a MEA 6. FIG. 9 is a schematic view showing the MEA
6. Numeral 91 denotes the catalyst, numeral 92 denotes the carbon,
numeral 93 denotes the alumina particles, and numeral 94 denotes
the thin film electrolyte formed on the alumina particles.
[0097] In addition, a unit cell 6 was produced using the MEA 6.
Example 7
[0098] Example 7 is directed to an exemplary case in which a
catalyst layer includes catalyst particles and other porous
materials, the catalyst is present as one piece inside pores of the
porous materials, and the porous materials serve as a substrate to
which a thin film electrolyte is bonded.
[0099] A trialkoxysilane compound
H.sup.+SO.sub.3--(CH.sub.2).sub.2(CF.sub-
.2).sub.14(CH.sub.2).sub.2Si(OCH.sub.3).sub.3 was pressed into
pores of a 100 .mu.m thick porous glass plate, which was an
inorganic porous body, having 0.004 to 0.02 .mu.m pores. Then, the
porous glass plate was burned in the air at 120.degree. C. so as to
cause dealcoholization, thereby introducing an electrolyte into the
pores.
[0100] Onto a surface of this porous glass plate, a catalyst paste
containing platinum black and a Flemion solution FSS-1
(manufactured by ASAHI GLASS CO., LTD., 9 wt % ethanol solution),
which was a perfluorocarbon sulfonic acid solution, was applied and
then dried in a nitrogen atmosphere at 60.degree. C. After drying,
onto a surface opposite to the paste-applied surface, a catalyst
paste containing platinum ruthenium black and the Flemion solution
was applied and then dried in a nitrogen atmosphere at 60.degree.
C.
[0101] FIG. 10 is a schematic view showing a catalyst layer.
Numeral 101 denotes an electrolyte part, numeral 102 denotes an
anode catalyst layer, and numeral 103 denotes a cathode catalyst
layer. Numeral 104 denotes the porous glass plate, whose pores have
an inner surface provided with a thin film electrolyte 105. Numeral
106 indicates the platinum ruthenium black forming the anode
catalyst layer, and numeral 107 indicates the platinum black
forming the cathode catalyst layer.
[0102] The outer sides of the catalyst layers were sandwiched by
carbon papers from both sides in a manner similar to that in
Example 1, thus forming a MEA 7. In addition, a unit cell 7 was
produced using the MEA 7.
Example 8
[0103] Example 8 is directed to an exemplary case in which a
catalyst layer includes catalyst particles, an electron conducting
substance and other porous materials, the catalyst is present as
one piece inside pores of the porous materials, and the porous
materials serve as a substrate to which a thin film electrolyte is
bonded.
[0104] A trialkoxysilane compound
H.sup.+SO.sub.3--(CH.sub.2).sub.2(CF.sub-
.2).sub.14(CH.sub.2).sub.2Si(OCH.sub.3).sub.3 was pressed into
pores of a 100 .mu.m thick porous glass plate, which was an
inorganic porous body, having 0.004 to 0.02 .mu.m pores. Then, the
porous glass plate was burned in the air at 120.degree. C. so as to
cause dealcoholization, thereby introducing an electrolyte into the
pores of the porous glass plate.
[0105] Next, the pores of the porous glass plate were filled with a
catalyst paste containing platinum carrying carbon TEC10E50E
(manufactured by Tanaka Kikinzoku Kogyo K.K.) or platinum ruthenium
carrying carbon TEC61E54 (manufactured by Tanaka Kikinzoku Kogyo
K.K.), a PTFE dispersion and ion exchanged water.
[0106] The porous glass plate had the pores in which an electrolyte
was formed, and the catalyst paste was filled in the pores and
dried. Further, it was formed in one piece with a gas diffusion
layer by the method described in Example 1 so as to obtain a MEA 8.
A unit cell 8 was produced using the MEA8.
[0107] FIG. 11 is a schematic view showing the MEA. Numeral 111
denotes an electrolyte part, numeral 112 denotes an anode catalyst
layer, and numeral 113 denotes a cathode catalyst layer. Numeral
114 denotes the porous glass plate, whose pores have an inner
surface provided with a thin film electrolyte 115. Numeral 116
indicates the platinum ruthenium carrying carbon forming the anode
catalyst layer, and numeral 117 indicates the platinum carrying
carbon forming the cathode catalyst layer.
Example 9
[0108] Example 9 is directed to a case of using functional groups
other than a sulfonic acid as the ion-conducting functional group.
Platinum carrying carbon or platinum ruthenium carrying carbon in
which a functional group of the carrier carbon surface was treated
according to the method described in Example 5 was immersed in a
toluene solution containing a silane compound for 1 hour according
to the method described in Example 1. Table 1 shows the silane
compounds used here.
1TABLE 1 Functional group HPO.sub.4 NH.sub.2 OH Silane Compound
Formula (6) Formula (7) Formula (8) Molecular weight 151 80 131 MEA
MEA 9 MEA 10 MEA 11
[0109] A catalyst layer of the MEA 9 was obtained using
vinyltrimethoxysilane (Sila-Ace S210; manufactured by CHISSO
CORPORATION, represented by the formula (6) below), which was the
silane compound, as follows. A molecular thin film from the silane
compound was formed on the catalyst carrying carbon according to
the method described in Example 1 and then heated in a phosphoric
acid solution. A catalyst layer of the MEA 10 was obtained using
3-aminopropyltrimethoxysilane (Sila-Ace S360; manufactured by
CHISSO CORPORATION, represented by the formula (7) below), which
was the silane compound. A catalyst layer of the MEA 11 was
obtained using triglycidoxypropyltrimethoxysilane (KBM-403;
manufactured by Shin-Etsu Chemical Co., Ltd., represented by the
formula (8) below), which was the silane compound, as follows.
After a molecular thin film was formed according to the method
described in Example 1, the catalyst carrying carbon was washed in
dilute sulfuric acid so as to cleave an epoxy ring, whereby an OH
group was introduced.
CH.sub.2.dbd.CH--Si(OCH.sub.3).sub.3 (6)
H.sub.2N--CH.sub.2CH.sub.2CH.sub.2--Si(OCH.sub.3).sub.3 (7) 6
[0110] After the introduction of the functional group, the catalyst
carrying carbon was washed using a toluene anhydride solution,
dried in a nitrogen atmosphere and then mixed with ion exchanged
water and a PTFE dispersion, thus preparing a catalyst paste, which
was then applied to water-repellent carbon papers and cut into a
predetermined size so as to be formed in one piece with an
electrolyte part. MEAs produced as above were the MEA 9, the MEA 10
and the MEA 11, respectively. Further, a unit cell 9, a unit cell
10 and a unit cell 11 were produced using the respective MEAs.
Comparative Example
[0111] Comparative Example shows an exemplary case of using a
perfluorocarbon sulfonic acid as an electrolyte in a catalyst
layer. The catalyst layer used here was platinum or platinum
ruthenium carried by Ketjen Black EC. 50 wt % platinum having a
mean particle diameter of 3 nm carried by Ketjen Black EC (trade
name; manufactured by AKZO Chemie, the Netherlands) having a mean
primary particle diameter of 30 nm was used for a cathode, while 25
wt % platinum having a mean particle diameter of 3 nm and 25 wt %
ruthenium having a mean particle diameter of 3 nm that were carried
by the same Ketjen Black EC were used for an anode.
[0112] These catalyst carrying particles and a polymer electrolyte
were mixed to form a catalyst paste. In this case, the weight ratio
of carbon in the catalyst carrying particles to the polymer
electrolyte was 1:1. The polymer electrolyte used here was an
ethanol/isopropanol mixed solution of Nafion (manufactured by
DuPont.), which was a perfluorocarbon sulfonic acid polymer.
[0113] Next, the catalyst paste was printed on an electrolyte part
that had been produced by filling a thin film electrolyte in pores
of an alumina membrane filter described in Example 1. The catalyst
paste was formed in one piece with carbon papers according to the
method described in Example 1, thus producing a MEA 12. In
addition, a unit cell 12 was produced using the MEA 12.
[0114] Evaluation of Cell Performance
[0115] The produced unit cells 1 to 12 were individually stacked by
the method described in Example 1 and evaluated. 2 mol/L methanol
aqueous solution at 60.degree. C. was supplied as a fuel at 2 cc
per minute, and the air was supplied under the condition of a cell
temperature of 60.degree. C. and an air utilization factor of 30%.
The air outlet was pressurized at 2 atm.
[0116] Table 2 shows OCV and mean unit cell voltages at current
densities of 200 mA/cm.sup.2 and 500 mA/cm.sup.2.
2 TABLE 2 Cell No. OCV 200 mA/cm.sup.2 500 mA/cm.sup.2 Unit cell 1
782 501 367 Unit cell 2 788 499 363 Unit cell 3 788 498 360 Unit
cell 4 802 522 388 Unit cell 5 798 512 378 Unit cell 6 784 505 370
Unit cell 7 798 494 360 Unit cell 8 800 496 365 Unit cell 9 788 498
373 Unit cell 10 782 484 370 Unit cell 11 780 480 363 Unit cell 12
752 426 154 (Comp. Ex.) (unit: mV)
[0117] Even in the case of using 10 mol/L methanol aqueous
solution, which was a high concentration methanol aqueous solution,
the OCV and the voltages at a current density of 200 mA/cm.sup.2 as
shown in Table 3 were obtained.
3 TABLE 3 Cell No. OCV 200 mA/cm.sup.2 Unit cell 1 392 209 Unit
cell 2 390 205 Unit cell 3 388 202 Unit cell 4 401 216 Unit cell 5
398 212 Unit cell 6 396 204 Unit cell 7 402 204 Unit cell 8 400 202
Unit cell 9 396 210 Unit cell 10 392 198 Unit cell 11 388 196 Unit
cell 12 (Comp. Ex.) 287 89 (unit: mV)
[0118] Even when using the high concentration methanol aqueous
solution, the electrolyte in the catalyst layer did not elute very
much. Accordingly, an effective reaction area, which was a contact
of the catalyst, the fuel and the hydrogen ion conductor,
increased, and thus, the voltage rose.
[0119] Incidentally, although methanol was used as an example of
the fuel in the present example, the similar result was obtained
also by using a hydrocarbon fuel such as hydrogen, ethanol,
ethylene glycol, dimethyl ether, isopropanol, glycerin, methane or
dimethoxymethane, or a mixture thereof. Further, the liquid fuels
also may be mixed together in advance and supplied in the form of
vapor.
[0120] Moreover, the structure of the gas diffusion layer of the
present invention is not limited to the electrically conductive
carbon paper illustrated in the above examples. Instead, other
electrically conductive carbon cloths and metal meshes also can be
used effectively.
[0121] The invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The embodiments disclosed in this application are to be
considered in all respects as illustrative and not restrictive, the
scope of the invention being indicated by the appended claims
rather than by the foregoing description, all changes that come
within the meaning and range of equivalency of the claims are
intended to be embraced therein.
* * * * *